1. Field of the Invention
The present invention generally relates to circuits, systems and methods for extending a measurement detection range of an inspection system used for inspecting a specimen. More specifically, the present invention relates to circuits, systems and methods for extending the measurement detection range of an inspection system by addressing the anode saturation of a photomultiplier tube (PMT) as a limiting factor of the measurement detection range.
2. Description of the Related Art
Fabricating semiconductor devices, such as logic, memory and other integrated circuit devices, typically includes processing a specimen such as a semiconductor wafer using a number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that typically involves transferring a pattern to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process, and thus, higher profits. Inspection has always been an important part of semiconductor fabrication. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices. For instance, detecting defects of decreasing size has become increasingly necessary, since even relatively small defects may cause unwanted aberrations in the semiconductor device, and in some cases, may cause the device to fail.
Many different types of inspection tools have been developed for the inspection of semiconductor wafers, including optical and E-beam systems. Optical inspection tools may be generally characterized into dark-field and bright-field inspection systems. Dark-field systems are typically known for having a relatively high detection range. For instance, dark-field systems detect the amount of light that is scattered from the surface of a specimen when an incident beam is supplied to the specimen at a normal or oblique angle. The amount of scattered light detected by the system generally depends on the optical characteristics of the spot under inspection (e.g., the refractive index of the spot), as well as any spatial variations within the spot (e.g., uneven surface topologies). In the case of dark-field inspection, smooth surfaces lead to almost no collection signal, while surfaces with protruding features (such as patterned features or defects) tend to scatter much more strongly (sometimes up to six orders of magnitude or more). Bright-field inspection systems direct light to a specimen at a particular angle and measure the amount of light reflected from the surface of the specimen at a similar angle. In contrast to dark-field systems, the variations in the reflected signal collected by a bright-field system are generally no more than about two orders of magnitude.
In addition, most inspection tools are designed to inspect either unpatterned or patterned semiconductor wafers, but not both. Since the tools are optimized for inspecting a particular type of wafer, they are generally not capable of inspecting different types of wafers for a number of reasons. For example, many unpatterned wafer inspection tools are configured such that all of the light collected by a lens (or another collector) is directed to a single detector that generates a single output signal representative of all of the light collected by the lens. Therefore, light scattered from patterns or features on a patterned wafer will be combined with other scattered light (e.g., from defects). In some cases, the single detector may become saturated, and as a result, may not yield signals that can be analyzed for defect detection. Even if the single detector does not become saturated, the light scattered from patterns or other features on the wafer cannot be separated from other scattered light thereby hindering, if not preventing, defect detection based on the other scattered light.
Tools used for inspecting patterned wafers generally employ at least two detectors for improved spatial resolution. However, the detectors used in patterned wafer inspection tools may also become saturated, especially when imaging with a dark-field system. As noted above, dark-field scattering signals obtained from a patterned wafer may vary by six orders of magnitude (or more) due to the variation in surface topology from smooth surface regions (which appear dark) to highly textured regions (which appear bright). It is often difficult, especially with detection systems operating at high data rates, to collect meaningful signals from both the very dark and the very bright areas of the substrate being inspected without “on-the-fly” adjustment.
Most optical inspection tools are limited in either detection range, detection sensitivity, or both. For example, inspection tools employing high-gain detectors to obtain higher detection range may be incapable of detecting smaller (i.e., low light) signals. On the other hand, inspection tools with lower-gain detectors may achieve greater sensitivity at the cost of reduced detection range. In other words, although lower gain detectors may be capable of detecting smaller signals, they may become saturated when larger signals are received. Other factors tend to limit the detection range, in addition to detector gain. For example, further limitations may be imposed by the amplification circuitry or the fast analog-to-digital converters used to convert the scattered output signals into a format suitable for signal processing.
One possible solution to this problem is to apply non-linear amplification to the output signal of a detector, in order to emphasize the low-amplitude signal range. An approach of this sort is described by Wolf in U.S. Pat. No. 6,002,122, whose disclosure is herein incorporated by reference. In the method described by Wolf, the output signal from a photomultiplier tube (PMT) is processed by a logarithmic amplifier and gain correction mechanism. Wolf emphasizes the low-amplitude signal range by changing the PMT gain “on-the-fly” (by changing the bias potentials supplied to the dynodes) to avoid anode saturation, a common detection range limitation of PMT detectors. Although this approach may provide improved visibility of small-signal defects in the dark-field image, it does nothing to extend the overall detection range of the inspection system. In addition, the “on-the-fly” gain modulation disclosed by Wolf causes the PMT to be operated in a highly non-linear way, thus requiring complex (and expensive) drive electronics and sophisticated calibrations to compensate for the non-linear and transient effects.
Another approach to extend the detection range of an inspection system is to utilize two or more detectors with separate detection channels. An approach of this sort is described by Almogy et al. in U.S. Patent Application No. 2003/0058433, whose disclosure is herein incorporated by reference. Almogy describes a defect detection system that utilizes at least two detectors. One of the detectors is optimized for high sensitivity, while the other is designed to have a high saturation level, typically at the expense of sensitivity. The light scattered from a specimen is split among the detectors with the addition of various optical components. Though Almogy is able to extend the detection range, Almogy does so by requiring multiple detectors with additional optics and electronic circuitry, all of which consume additional space, increase complexity, and incur higher cost.
Therefore, a need remains for improved circuits and methods for extending the detection range of a wafer inspection system. Preferably, such improved circuits and methods would provide significant measurement range extension without the complexity and cost of real-time gain adjustment, as required by Wolf, or the additional detectors, optics and electronic circuitry required by Almogy. In addition, an improved inspection system would extend the detection range without sacrificing throughput or sensitivity. In some cases, the improved inspection system may be used for inspecting both patterned and unpatterned wafers.